Various types of sensor for detecting pressure in a fluid are known. The most conventional type uses an elastic diaphragm equipped with strain gauge (sometimes called piezoresistive) elements. However, although one of the lowest cost pressure sensors currently made, these sensors tend to be relatively large in size, and have a mechanical structure which although producible by mass-production photolithographic methods is still relatively complex and expensive to make. They also have a certain degree of fragility and require calibration and temperature compensation before they can be used.
Pressure sensors comprising a flexible, resilient diaphragm and which utilize capacitance in their action are also known. For example, U.S. Pat. No. 4,204,244 discloses a pressure sensor, which can be used in automobile internal combustion engines, comprising a flexible diaphragm and a rigid base member separated at their circumferences by an annular wall. The diaphragm and the base member each carry an electrode, and a reference vacuum is stored in the enclosure formed by the diaphragm, base and wall, changes in external pressure causing the diaphragm to flex so changing the capacitance of the sensor.
Other pressure sensors, used most often in water and commonly known as hydrophones, employ a piezoelectric solid as their active element. However, these sensors can only be used for pressure values which change rapidly, on a timescale of seconds or less, as they suffer from rapid zero drift.
U.S. Pat. No. 4,924,701 discloses a pressure sensor for use in high pressure environments, such as underground oil reservoirs, for detecting small changes in pressure. The sensor comprises first and second capacitors defined by fixed first and second capacitor plates on either side of a common capacitor plate, with a gaseous dielectric medium between the plates. The plates and gaseous medium are enclosed in a housing including a diaphragm which flexes with changes in pressure of the fluid being measured, thus causing the gaseous dielectric medium to compress or expand changing its dielectric constant and so the total capacitance of the system. The substrate carrying the common capacitor plate flexes with changes in temperature, so allowing such changes to be detected via the relative change in the capacitance of the first and second capacitors.
DE 3023218A1 discloses a capacitive pressure sensor comprising two electrically conductive coverings separated by an elastic electrically insulating layer containing gas inclusions. As pressure increases, the insulating layer and gas inclusions are compressed, increasing the capacitance of the sensor. The gas inclusions are used to reduce the elastic modulus of the dielectric, and so increase the sensitivity of the sensor. In one embodiment, a stretched polypropylene film is used as the insulating layer, and in another ground rubber particles are used. No examples of use of the sensor are given.
US 2004/0159158 A1 describes a similar capacitive pressure sensor, comprising a pair of conductive plates separated by a compressible dielectric, for use in sensing the pressure inside a car tire. Use of a separate temperature sensor, such as a anemometer, semiconductor device, chemical device or thermistor, to allow for temperature compensation is suggested. Techniques for correcting for centripetal force are also disclosed. Silicone foam material, rubber material, synthetic rubber material, neoprene, polyurethane foam, and polytetrafluoroethylene (PTFE) foam are suggested as suitable dielectrics. In an exemplary embodiment, silicone foam rubber is used.
U.S. Pat. No. 4,545,254 discloses a further capacitive sensor in which the electrodes are separated by a dielectric material selected from specific pyrochlore ferroelectric ceramic materials. It is stated that the sensor is suitable for use in cryogenic temperatures, but no further indication is given as to intended or suitable uses of the sensor.
U.S. Pat. No. 3,787,764 discloses a capacitive pressure sensor, comprising a pair of electrodes separated by a solid dielectric material, for use in measuring fluid pressure in a container. The capacitor is used to measure fluid pressures up to 35,000 psi. In the exemplified embodiments, an ionic crystal of calcium fluoride is used as the solid dielectric material.
U.S. Pat. No. 4,459,856 discloses a capacitive pressure transducer system comprising a reference capacitor and a pressure sensitive capacitor. The capacitors both comprise a first and second electrically conductive layers separated by a compressible dielectric, compression of the reference capacitor dielectric being restrained by an insulative wall portion. The capacitors form part of a circuit which provides a voltage output correlated to the difference in capacitance between the two capacitors.
US 2004/0164868 describes a carbon dioxide fire extinguishing device comprising a capacitive measuring device for detecting gas loss from the carbon dioxide pressure tank. The capacitive measuring device comprises a probe, which preferably extends the entire height of the pressure vessel, and comprises two coaxial tubular electrodes, with liquid, gaseous or supercritical carbon dioxide forming the intermediate dielectric.
KR20040100001 describes a breathing apparatus comprising a high pressure air cylinder, a pressure sensor for measuring air pressure, and a transmission unit for transmitting the remaining pressure to a wireless display unit on the respiration unit interface.
GB 2111749 describes a power capacitor comprising a plurality of capacitor elements. The capacitor elements comprise first and second foils wound together, a solid dielectric material comprising polymer films separating the first and second foils from each other. The polymer film is preferably polypropylene. Other polymers that can be used are polyethylene, copolymerisates of ethylene and propylene and polymethylpentane, polycarbonate, polyethyleneglycolterephthalate, and polyimide.